1. Technical Field
The present invention relates to the measurement of the amplitude and angular position of an untrue running in a revolving system.
The invention may be particularly, but not exclusively applied to measuring apparatus for providing the necessary indications for balancing an aircraft jet engine.
2. Prior Art
The verification of the balance of the jet engines of an aircraft is a control operation generally effected on a track. For this purpose, the jet engines of the aircraft are each provided with a vibratory acceleration sensor on a bearing and a speed of rotation and position sensor. These sensors are permanently mounted and the detection and measurement of a possible untrue running is realised by connecting a suitable apparatus to the output signals of the sensors. It is known to use an apparatus which directly provides an indication of the amplitude of an untrue running and an indication of the angular position of the imbalance causing the untrue running. In such an apparatus, the signal from the vibratory acceleration sensor, before amplification and a first filtering, undergoes two successive integrations and is applied to a tracking narrow bandpass filter centred permanently on a frequency corresponding to the rate of rotation measured by the other sensor. The output signal of the tracking filter is representative of the amplitude of any untrue running. The phase difference between this output signal and that provided by the position and speed of rotation sensor is determined in order to provide information representative of the angular position of the imbalance causing the untrue running.
In practice, the use of the information provided by this known measuring apparatus appears to pose several problems.
First of all, in certain cases, it is necessary for the operator to correct the phase difference measured by the apparatus by an angle representing any angular displacement between the positions of the vibratory force sensor and the speed of rotation and position sensor. This displacement is not necessarily the same on all types of engine. Further, balance controls are not generally carried out by highly qualified personnel and are accomplished in difficult conditions (outside and in noise). Errors in the sign of the correction of the measured phase difference and in the conversion of the measured phase difference to a particular blade to which a mass must be added for example are frequently made.
Moreover, it is established that the measured phase difference between the signal representing the amplitude of the untrue running and the signal from the speed of rotation and position sensor, after correction for the possible angular shift between the positions of the sensors, does not always correspond to the real value of the angular position of the imbalance causing the untrue running.
In effect, another error is introduced by the natural phase difference of the electronic circuits of the measuring apparatus. It can be shown that a supplementary error is introduced by a phase difference which is a "mechanical" function of the test conditions. It is actually established that different phase differences are measured on engines according to whether they are mounted on the bench or on an aircraft, even though the engines may be identical in all other respects. It has equally been demonstrated that the speed of rotation in the course of a test has a far from negligible influence.
These difficulties explain why in practice the balancing of an engine is frequently done by trial and error and without necessarily arriving at a perfect result.